Abstract:Transfer of angular momentum from a spin-polarized current to a ferromagnet provides an efficient means to control the dynamics of nanomagnets. A peculiar consequence of this spin-torque, the ability to induce persistent oscillations of a nanomagnet by applying a dc current, has previously been reported only for spatially uniform nanomagnets. Here we demonstrate that a quintessentially nonuniform magnetic structure, a magnetic vortex, isolated within a nanoscale spin valve structure, can be excited into persistent microwave-frequency oscillations by a spin-polarized dc current. Comparison to micromagnetic simulations leads to identification of the oscillations with a precession of the vortex core. The oscillations, which can be obtained in essentially zero magnetic field, exhibit linewidths that can be narrower than 300 kHz, making these highly compact spin-torque vortex oscillator devices potential candidates for microwave signalprocessing applications, and a powerful new tool for fundamental studies of vortex dynamics in magnetic nanostructures. Pribiag et al.1A spin-polarized electron current can apply a torque on the local magnetization of a ferromagnet. This spin-transfer effect 1,2 provides a new method for manipulating magnetic systems at the nanoscale without the application of magnetic fields and is expected to lead to future data storage and information processing applications 3 . Experiments have demonstrated that spin-torque can be used to induce current-controlled hysteretic switching, as well as to drive persistent microwave dynamics in spin-valve devices 3,4,5,6,7,8,9,10,11,12 . While it is known that spin-torque switching of a magnetic element can sometimes occur via non-uniform magnetic states 13 , a central remaining question is whether spin-torque can be used to efficiently excite steady-state magnetization oscillations in strongly non-uniform magnetic configurations in a manner suitable for fundamental investigations of nanomagnetic dynamics and improved device performance. A relatively simple type of non-uniform magnetic structure is a magnetic vortex, the lowest-energy configuration of magnetic structures just above the singledomain length scale 14 . Previous studies, typically performed on single-layer permalloy (Py) structures, focused on the transient or resonant response of a magnetic vortex to an applied magnetic field and identified the lowest excitation mode of a vortex as a gyrotropic precession of the core 15,16,17,18 . It has also been demonstrated that the vortex core polarization can be efficiently switched by short radio-frequency magnetic field pulses 19 . Recently, the spin-transfer effect has been used to drive a magnetic vortex into resonant precession by means of an alternating current incident on a single Py dot 20 . Here we report by means of direct frequency-domain measurements that a dc spinpolarized current can drive highly coherent gigahertz-frequency steady-state oscillations of the magnetic vortex in a nanoscale magnetic device. The high sensitivity of ou...

In low resistance-area product MgO magnetic tunnel junction nanopillars, we observe high integrated power (up to 43nW), narrow linewidth (down to 10MHz), spin transfer induced microwave emission at frequencies up to 14GHz due to precession of the free layer magnetization at room temperature. Although all devices were fabricated on the same wafer, they present bimodal transport and precessional characteristics. The devices in which the narrowest linewidths were observed exhibited low resistance and tunneling magnetoresistance (30%), while maintaining large integrated power.

We discuss aspects of Andreev reflection ͑AR͒ measurements in normal metal-superconductor ͑N-S͒ and ferromagnet-superconductor ͑F-S͒ devices. We describe the analytical model used to quantify spin polarization from the conductance measurements and discuss the validity of this simple model using parabolic bands as simple surrogates for real band structures. We present ͑AR͒ measurements of spin polarization in a Cu-Pt-Pb and Co-Pt-Pb lithographically fabricated nanocontact systems where a scattering layer of has been deliberately added to the interface to enable the study of the effect of pair-breaking scattering on AR conductance and spin polarization. We compare these results to the previously published results from clean Cu-Pb and Co-Pb devices and argue that the measurements in devices with the Pt layer can be explained by the presence of inelasticscattering-induced pair-breaking effects. We modify the analytical model to include this effect and show that in some instances, it may be impossible to distinguish between the effects of a finite spin polarization and inelastic scattering. This has implications for AR measurements of spin polarization at disordered or poorly formed F-S interfaces.

Tailoring Gilbert damping of metallic ferromagnetic thin films is one of the central interests in spintronics applications. Here we report a giant Gilbert damping anisotropy in epitaxial Co50Fe50 thin film with a maximum-minimum damping ratio of 400 %, determined by broadband spin-torque as well as inductive ferromagnetic resonance. We conclude that the origin of this damping anisotropy is the variation of the spin orbit coupling for different magnetization orientations in the cubic lattice, which is further corroborate from the magnitude of the anisotropic magnetoresistance in Co50Fe50.In magnetization dynamics the energy relaxation rate is quantified by the phenomenological Gilbert damping in the Landau-Lifshits-Gilbert equation [1], which is a key parameter for emerging spintronics applications [2][3][4][5][6]. Being able to design and control the Gilbert damping on demand is crucial for versatile spintronic device engineering and optimization. For example, lower damping enables more energy-efficient excitations, while larger damping allows faster relaxation to equilibrium and more favorable latency. Nevertheless, despite abundant approaches including interfacial damping enhancement [7-9], size effect [10,11] and materials engineering [12][13][14], there hasn't been much progress on how to manipulate damping within the same magnetic device. The only well-studied damping manipulation is by spin torque [15][16][17][18], which can even fully compensate the intrinsic damping [19,20]. However the requirement of large current density narrows its applied potential.An alternative approach is to explore the intrinsic Gilbert damping anisotropy associated with the crystalline symmetry, where the damping can be continuously tuned via rotating the magnetization orientation. Although there are many theoretical predictions [21][22][23][24][25], most early studies of damping anisotropy are disguised by two-magnon scattering and linewidth broadening due to field-magnetization misalignment [26][27][28][29]. In addition, those reported effects are usually too weak to be considered in practical applications [30,31].In this work, we show that a metallic ferromagnet can exhibit a giant Gilbert damping variation by a factor of four along with low minimum damping. We investigated epitaxial cobalt-iron alloys, which have demonstrated new potentials in spintronics due to their ultralow dampings [32,33]. Using spin-torque-driven and inductive ferromagnetic resonance (FMR), we obtain a fourfold (cubic) damping anisotropy of 400% in Co 50 Fe 50 thin films between their easy and hard axes. For each angle, the full-range frequency dependence of FMR linewidths can be well reproduced by a single damping parameter α. Furthermore, from first-principle calculations and temperature-dependent measurements, we argue that this giant damping anisotropy in Co 50 Fe 50 is due to the variation of the spin-orbit coupling (SOC) in the cubic lattice, which differs from the anisotropic density of state found in ultrathin Fe film [30]. We support our conclu...